Water-absorbable polyacrylic acid resin powder, and process for production thereof

Abstract

A process according to the present invention for production of water-absorbable polyacrylic acid resin powder, including: (a) polymerizing an acrylic acid-based monomer aqueous solution containing bubbles, and (b) drying a hydrogel crosslinked polymer thus obtained in the step of polymerizing, further includes (c) lowering solubility of dissolved gas in a monomer aqueous solution in the presence of a surfactant and/or a dispersing agent, so as to generate the bubbles in the acrylic acid-based monomer aqueous solution. With this arrangement, it is possible to provide white water-absorbable resin with better water absorbing rate, keeping or without significantly losing the other properties (permeability potential, bulk specific gravity, surface tension, absorbency against pressure, impact resistance, etc.)

The invention claimed is: 
     
       1. Water-absorbable resin powder being water-absorbable polyacrylic acid resin powder in which particles having a particle diameter of 850 μm to 150 μm accounts for 95 wt % or more, and having an internal cell rate of 2.8% to 6.6%, the internal cell rate being defined by the following equation:
   (internal cell rate [%]={(Real Density [g/cm 3 ])−(Apparent Density [g/cm 3 ])}/(Real Density [g/cm 3 ])×100,
 
 the water-absorbable resin powder having a degree of whiteness of 68 or more [WB value] as measured with use of a spectral colorimeter. 
 
     
     
       2. The water-absorbable resin powder as set forth in  claim 1 , containing a surfactant and/or a dispersing agent inside thereof, wherein:
 the water-absorbable resin powder has a surface tension of 60 [mN/m] or greater, and a particle surface being coated with a/the surfactant. 
 
     
     
       3. The water-absorbable resin powder as set forth in  claim 1 , wherein a saline flow conductivity thereof is 20 [×10 −7 ·cm 3 ·sec·g −1 ] or greater, and a water absorbing rate (FSR) thereof is 0.25 [g/g/sec] or greater. 
     
     
       4. The water-absorbable resin powder as set forth in  claim 1 , wherein an absorbency without pressure (CRC) thereof is 25 [g/g] or more, and an absorbency against pressure (AAP) under load of 50 [g/cm 2 ] thereof is 15 [g/g] or more. 
     
     
       5. The water-absorbable resin powder as set forth in  claim 1 , wherein an absorbency without pressure (CRC) thereof is 25 [g/g] or more, and a saline flow conductivity thereof is 20 [×10 −7 ·cm 3 ·sec·g −1 ] or greater. 
     
     
       6. The water-absorbable resin powder as set forth in  claim 1 , further comprising:
 one or more kinds of permeability improving agents selected from the group consisting of multivalent metal cations, polyamine polymers and water insoluble fine particles. 
 
     
     
       7. The water-absorbable resin powder as set forth in  claim 1 , further comprising a surfactant. 
     
     
       8. The water-absorbable resin powder as set forth in  claim 1 , further comprising p-methoxy phenol by 5 ppm to 60 ppm. 
     
     
       9. The water-absorbable resin powder as set forth in  claim 1 , further comprising one or more kinds of additives selected from the group consisting of chelating agents, α-hydroxy carboxylic acids, and inorganic or organic reducing agents. 
     
     
       10. The water-absorbable resin powder as set forth in  claim 1 , wherein
 a water absorbing rate index there of is 90 or more, water absorbing rate index being defined by the following equation:
   (Water Absorbing Rate Index)=(FSR [g/g/sec])×(Bulk Specific Gravity [g/cm 3 ])×(Weight Average Particle Diameter [μm]),
 
 
 where FSR is a water absorbing rate for saline so as to swell 20-fold, and 
 a bulk specific gravity thereof is in a range of 0.6 to 0.8 [g/cm 3 ]. 
 
     
     
       11. The water-absorbable resin powder as set forth in  claim 1 , wherein a surface tension thereof is 60 [mN/m] or greater. 
     
     
       12. The water-absorbable resin powder as set forth in  claim 1 , wherein a bulk specific gravity thereof is in a range of 0.58 to 0.80 [g/cm 3 ].